Digital communications are widely used in personal, commercial, industrial, military and other settings. In a typical communication session, a digital message is represented with electromagnetic signals, such as electrical voltages or modulated radio waves, that can be transmitted across wires, optical fibers, wireless media, or the like. Some digital communications take place over point-to-point links (such as an optical fiber or other wired connection) that can be readily secured by limiting physical access to the communications medium. Other communications occur over public or private networks (e.g., the Internet) or over wireless media where physical security is much more difficult.
Wireless and other physically-accessible networks are generally much easier to implement than secure dedicated links. The convenience of the Internet and various wireless networks, for example, has led to widespread use of such networks in many different settings. This openness, however, can lend itself to various types of security threats. Unauthorized listeners, for example, may be able to “listen in” on data communications that are not encrypted or otherwise obscured. For this reason, sensitive data is often encrypted or otherwise obscured prior to transmission across a public or other unsecure network.
Another type of security threat is a so-called “denial of service” (DOS) attack, in which authorized users are prevented from gaining access to a service that is available on a network. In a DOS attack, the attacker generally overwhelms the service with unnecessary data so that resources are not available for legitimate users. A typical DOS attack may involve numerous computers simultaneously requesting access to the service, thereby consuming processing and communications resources that would otherwise be available for rightful users. DOS attacks are particularly bothersome when the attacker knows the correct preamble, synchronization or other data values that are typically provided by legitimate users of the service. Such data may be obtained, for example, by capturing a legitimate message and then re-transmitting (or “replaying”) the same data to the service at a later time. DOS attacks may be mounted against web sites or other network services, as well as against cellular telephone systems (e.g., by overwhelming a base station or other resource with new requests for access or other data), military or public safety communications systems, or any other services that are accessible by unsecured wired or wireless media.
It is therefore desirable to create systems and techniques to prevent denial-of-service attacks. It is further desirable that such systems and techniques be effective, yet computationally manageable. These and other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background section.